Chlorinated polyolefins are chlorinated compounds obtained by chlorination of polyolefins such as polyethylene. Chlorinated polyolefins are commonly used as modifiers for ABS resins or polyvinyl chloride resins or as wire sheaths, but in recent years they are also coming into wider use as crosslinked rubbers or thermoplastic elastomers for automobile parts and industrial parts.
Because of the oil resistance, solvent resistance, weather resistance and flexibility characteristic of chlorinated polyolefins, they are also used as hoses or sheets, packings, automobile constant velocity universal joint boots and the like. Such uses also require additional properties such as satisfactory heat resistance, ozone resistance, low-temperature rubber elasticity, compression set and fatigue strength.
Chlorinated polyolefins have conventionally been produced by chlorinating polyolefin powder in aqueous suspension while controlling the reaction temperature, chlorine content, etc. Various proposals have been set forth for such aqueous suspension methods, in order to obtain a uniform chlorinated polyolefin powder and in order to prevent agglomerating during the chlorinated reaction and maintain the powder particle size.
Surfactants or inorganic substances are commonly added as anti-aggregation agents in order to prevent agglomerating of particles during the reaction. On the other hand, when a copolymer of ethylene and an α-olefin is used as the starting material, a higher proportion of the α-olefin tends to result in lower crystallinity, and the increased number of free chains promotes agglomerating during the chlorination reaction. A production method known as a solution to this problem comprises a first step of chlorination at a temperature lower than the crystal melting point of the polyolefin starting material, a second step of heat treatment in the absence of chlorine at a temperature higher than the crystal melting point, and a third step of chlorination up to the final chlorine content at a temperature below the temperature of the second step and below the crystal melting point (see, for example, Japanese Unexamined Patent Publication No. 3-66325).
However, the effect of satisfactorily preventing agglomerating and maintaining the powder particle size depends on preventing unwanted crystal residue or a relatively poor level of non-uniformity, and it has been difficult to achieve uniform chlorinated polyolefins with excellent properties by the method described above.
It is also well known that polymerized polyolefin powder has a given range of particle size distribution and that differences in molecular weight and density are found between the particle sizes. The uniformity of a chlorinated polyolefin depends primarily on the uniformity of the polyolefin starting material, and methods are known for obtaining uniform chlorinated polyethylene by adjusting the particle size of the polyolefin starting material (see, for example, Japanese Unexamined Patent Publication No. 8-59737).
However, while improvement toward a more uniform product is achieved by narrowing the particle size distribution of the polyolefin powder, the more serious problem of variation within each particle is not affected, and these obtained chlorinated polyolefins therefore remain unsatisfactory in several of their properties.